Calculating Your Due Date

For those women who find themselves pregnant unexpectedly, they may need some help calculating their due date. Calculating your due date is not exactly a science, but rather, is based upon averages. The average pregnancy lasts 40 weeks, or 280 days from the first date of your last menstrual period — also referred to as LMP — or 266 days from the day of conception.

The internet is a huge help in calculating your due date thanks to the sheer number of due date calculators out there. You can calculate your due date the old fashioned way by adding 280 to the first day of your LMP, or 266 to the day of conception — more days than you can count on all your fingers and toes! Since many women may not know the exact date of conception, using your last menstrual period as a guide may be preferable. Your doctor can assist you in calculating your due date at your first visit.

If you have irregular menstrual cycles, or don’t track them well, your doctor may perform an ultrasound to calculate the due date. Early on in a pregnancy, a transvaginal ultrasound is more commonly used because it gives your doctor a clearer look at the developing fetus. Although it may be uncomfortable, a transvaginal ultrasound is less painful than a regular vaginal exam. The ultrasound’s transducer is inserted into the vagina for a picture of the fetus and the amniotic sac, giving the doctor a better idea of when your due date is.

Even if you are definite on your date of conception or LMP, your doctor may simply give you a due “week” instead of a due date. This gives a little leeway in calculating your due date, instead of providing a specific due date. In reality, babies are rarely born on their actual due date — this only occurs about 5% of the time. The vast majority of babies are born within 14 days plus or minus of their due date. 80% are born within the ten days around the due date. Only about 10% of babies are born before 38 weeks gestation, and 8% are born after 42 weeks.

You may choose to take the guess work out of calculating your due date by scheduling an elective induction. If you must have a cesarean due to medical reasons or previous c-sections, you also get the benefit of scheduling your baby’s birthday as well. Of course, your baby may “decide” to come earlier than the date you set. These are options that you should fully research and discuss with your doctor before making a decision. It is important to be flexible with your due date, and prepare well in advance for your baby, and remember — he or she will come when they’re ready, whether you’re ready or not.